Optically anisotropic pitches that can be spun into carbon fibers have been produced previously by heat soaking an aromatic feedstock containing polycondensed aromatic (3, 4, 5, 6 and 7) rings or by heating a petroleum pitch containing larger aromatic rings. During the heat soaking treatment polycondensed aromatic rings will polymerize and condense into aromatic ring agglomerates called liquid crystals (mesophase) which are 100% optically anistropic when polished sections are examined by polarized light microscopy.
Highly anisotropic pitches prepared from aromatic feed or from petroleum pitch contain unreacted oils, often in substantial amounts (25-35%). These oils must be almost completely removed to produce a pitch with the desired rheological properties such as softening point and viscosity; which are critical parameters for successful spinning, oxidation, carbonization treatments of the green fiber in the production of high tensile strength carbon fibers.
Removal of the unreacted oil from the heat soaked feed mixture can be achieved by many methods including vacuum stripping the unreacted oil at the end of the heat soaking step. This can be carried out by using the same heat soaking reactor. Such a method of oil removal has been used effectively to prepare aromatic pitches from steam cracker tar, catalytic cracking bottom and coal by-products. The preparation of these pitches are described in the following U.S. patents and patent applications: Pat. No. 4,086,156 (1978); Ser. No. 225,060 (1981); Ser. No. 346,624 (1982); Ser. No. 346,623 (1982); Ser. No. 399,751 (1982); Ser. No. 399,472 (1982); and Ser. No. 399,702 (1982).
Another method of removing the unreacted oils calls for conducting the heat soaking under reduced pressure, where these unreacted oil are removed continuously during the heat soaking step. This procedure for pitch preparation is described in U.S. Pat. No. 4,271,006 (1981).
A further method for removing the unreacted oil from the heat soaking mixture is by injection of an inert gas at the bottom of the heat soaked mixture to volatilize the light, distillable oils. The oil stripping efficiency and rate of oil removal will, of course, be dependent on the design of the reactor and the distillate recovery system, the rate that the inert gas is passed into the mixture, the design of the sparger, as well as the rate of agitation. A major objective of the present invention is to make maximum utilization of the stripping gas in such a process.
U.S. Pat. No. 3,974,264 (McHenry) describes such a process for producing a pitch with a high mesophase content using a substantially shorter time by passing an inert gas through the heated pitch (350.degree.-450.degree. C.) during the formation of the mesophase at a rate of at least 0.5 SCFH/lb of pitch and generally at a rate of 0.7 to 5.0 SCFH/lb of pitch.
A later U.S. Pat. No. 4,209,500 (Chwastiak) describes the production of an aromatic pitch with high optical anisotropy by heating a petroleum pitch feed at 380.degree.-430.degree. C. and passing nitrogen through the heat soaked mixture at a rate of at least 4.0 SCFH/lb of pitch and up to 10.0 SCFH/lb of pitch. This patent asserts that an improved process for aromatic pitch production with 100% optical anisotropy is achieved by increasing the rate of which the nitrogen gas is passed into the heat soaked mixture for efficient stripping of the unreacted distillable oils thereby increasing the rate of mesophase formation.
As we indicated above, the degree of oil stripping from a heat soaked mixture depends on the rate of inert gas injection into the bottom of the reactor. It now has been found that the stripping of oils is also dependent on a number of other operating conditions.